Process for the preparation of tavaborole, its novel polymorphic forms and the polymorphs thereof

ABSTRACT

The invention relates to novel process for preparation of Tavaborole. The invention also relates to novel polymorphic forms of Tavaborole and process for preparation of those polymorphic forms. The invention also relates to process for purification of Tavaborole to obtain the Tavaborole in significantly high yield and substantially pure form.

RELATED APPLICATION

This is a continuation of the application Ser. No. 16/094,470, filed on Oct. 18, 2018, which is a national stage application under 35 U.S.C. 154(d)(4) and 35 U.S.C. 371 for PCT/IN2016/050246, filed on Jul. 21, 2016 and claimed priority under 35 U.S.C. 119(a) and 35 U.S.C. 365(b) to Indian Patent Application No. 201641013423, filed on Apr. 18, 2016.

FIELD OF THE INVENTION

The invention relates to novel process for preparation of Tavaborole. The invention also relates to novel polymorphic forms of Tavaborole and process for preparation of those polymorphic forms. The invention also relates to process for purification of Tavaborole to obtain the Tavaborole in significantly high yield and substantially pure form.

BACKGROUND OF THE INVENTION

The compound 5-fluoro-1,3-dihydro-1-hydroxy-2,1-benzoxaborole (Tavaborole) is represented by the formula 1:

Tavaborole is an oxaborole antifungal indicated for the topical treatment of onychomycosis of the toenails due to Trichophyton rubrum or Trichophyton mentagrophytes.

U.S. Pat. Nos. 5,880,188, 7,271,264, 8,115,026 reported various processes for the synthesis of oxaborole—specifically, Tavaborole, the contents of which are hereby incorporated as reference in their entirety.

General synthesis of Tavaborole is first described in the U.S. Pat. No. 5,880,188 (Prior art process—1). In this process oxaborole is made by converting an ortho-toluidine under Sandmeyer conditions to an O-substituted halogeno toluene which is then reacted with magnesium or alkyl lithium and the Grignard reagent or aryl lithium so formed is reacted with a borate ester to obtain a toluene boronic acid. This boronic acid is then reacted with NBS in CCl₄ solvent to give bromomethylbenzene boronic acid which is hydrolysed in alkali to give the benzaxoborole under acidic conditions.

In this process, the reported yield of the bromination stage is ˜15% and the overall yield is less than 5%, which makes the process impractical at a commercial scale production.

Another route for the preparation of oxaborole derivatives reported in the U.S. Pat. No. 5,880,188 (Prior art process—2) includes reacting benzaldehyde with p-toluenesulphonylhydrazide and reacting the product obtained subsequently with boron tribromide in the presence of a ferric or aluminium chloride which give the cyclized tosyl compound. The Tosyl compound on subsequent alkaline hydrolysis generates oxaborole compound.

In this process, the reported yield of tosylhydrazone to oxaborole conversion is less than 10%.

U.S. Pat. No. 7,271,264 presents another route of synthesis for Tavaborole (Prior art process—3). In this process, (2-bromo-5-fluoro-phenyl)-methanol is treated with ethylvinyl ether in presence of PPTS to give 1-bromo-2-(1-ethoxy-ethoxymethyl)-4-fluoro-benzene, which on further treatment with n-BuLi followed by reaction with trimethyl borate, and later with 1N HCl generates Tavaborole.

This process involves highly pyrophoric Butyllithium making it unsuitable for industrial scale synthesis.

U.S. Pat. No. 8,115,026 patent also describes a general route for the preparation of oxaborole compound including Tavaborole (Prior art process—4). In this process, orthohalo carbonyl compound is treated with bispinacolato diboran or pinacol borane in the presence of Pd catalyst, ligand and a base to give Pinacol borate which on oxidative cleavage with NaIO₄ generates boronic acid compound. Reduction of the carbonyl group of boronic acid compound with a suitable reducing agent produces the crude oxaborole compound and that is purified by column chromatography.

Even though the process avoids pyrophoric Butyllithium, it requires 30 mole percentage of expensive Palladium catalyst. In addition, this process involves column chromatographic purification of final compound and the overall yield is ˜21%.

Therefore, there is a need in the art for a new or improved method for the preparation of Tavaborole in significantly high yield compared to the prior art. In view of this, the present inventors provide a safe, cost-effective and industrially applicable process for the preparation of Tavaborole in significantly high yield.

OBJECTS OF THE INVENTION

Primary object of the invention is to provide novel process for the preparation of Tavaborole.

Another object of the invention is to provide novel polymorphic forms of Tavaborole.

Another object of the invention is to provide process for preparation of the novel polymorphic forms of Tavaborole.

Another object of the invention is to provide process for preparation of Tavaborole with high yield and purity.

Another object of the invention is to provide process for purification of Tavaborole to obtain highly purified Tavaborole.

A further object of the invention is to provide Tavaborole without the impurities represented as Impurity A and Impurity B.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides novel synthetic methods for the preparation of 5-fluoro-1,3-dihydro-1-hydroxy-2,1-benzoxaborole (Tavaborole 1).

In one aspect of the invention, synthesis of Tavaborole is reported which result in significantly high yield, greater than 80%.

In one aspect of the invention methods of purification of Tavaborole to obtain a substantially pure compound, whose purity is greater than or equal to 99% is reported.

In another aspect of the invention, Tavaborole obtained by the reported purification procedures of the invention has purity greater than or equal to 99% and is devoid of the major reaction impurities (A&B).

One of the processes for the preparation of Tavaborole according to the invention comprises three steps as depicted in Scheme—1.

Wherein

X is a member selected from Br, I, OTf;

R¹ is hydroxyl protecting group; and

R², R³ are members independently selected from H; substituted or unsubstituted C₁-C₈ alkyl; substituted or unsubstituted heteroalkyl C₁-C₈; substituted or unsubstituted aryl C₅-C₁₀; substituted or unsubstituted heteroaryl C₅-C₁₀; substituted or unsubstituted cycloalkyl C₃-C₈; substituted or unsubstituted heterocycloalkyl C₃-C₈; together with the atoms to which they are attached may be optionally joined to form a 4- to 8-membered ring.

-   -   i. The first step of the synthetic Scheme—1 comprises protecting         the primary hydroxyl moiety of the compound 2 with a suitable         hydroxyl protecting agent to generate compound 3.     -   ii. The second step comprises conversion of hydroxyl protected         compound 3 into a borate compound 4 (an ester or an acid—when         R², R³=‘H’) via a transition metal catalyzed cross-coupling         reaction.     -   iii. The third and the final step involves conversion of borate         compound 4 into Tavaborole (1)         -   a) either by simultaneous deprotection of hydroxyl             protecting group and borate group (ester or acid, when R²,             R³=‘H’) using a suitable deprotecting agent optionally in             the presence of an acid or a base; or         -   b) by sequential deprotection of hydroxyl protecting group             and boronate ester group under suitable conditions followed             by dehydrative cyclization in the presence of suitable acid             or a base.     -   Compound 4 can be converted to Tavaborole by an alternate route         via formation of 6A. The reaction stage includes oxidative         cleavage of compound 4 by treating with NaIO₄ or Pb(OAc)₄ in a         suitable solvent followed by cyclization in the presence of an         acid or a base.

In another embodiment of the invention Tavaborole is synthesized by the reaction depicted in Scheme—2.

wherein the substituents X, R², R³ are as defined above.

-   -   i. The first step of the process of Scheme—2 comprises,         conversion of the fluoro-phenyl compound 2 into borate compound         5 (ester or acid—when R², R³=‘H’) via a Nickel catalyzed         cross-coupling reaction. In this process, the reaction is         performed in the absence of any protection to primary hydroxyl         moiety of compound 2.     -   ii. Step-2 comprises conversion of borate compound 5 into         Tavaborole (1) under acid or base mediated hydrolysis         conditions.     -   iii. Step-3, alternatively comprises oxidative cleavage of the         borate compound 5 to generate compound 6 and subsequent         dehydrative cyclization into Tavaborole (1) in the presence of         an acid or a base.

In yet another aspect of the invention, crystalline polymorphic Forms I, II, III, IV & V of Tavaborole and processes for their preparation are reported.

In one embodiment the invention provides a process for preparation of 5-fluoro-1,3-dihydro-1-hydroxy-2,1-benzoxaborole (Tavaborole) comprising the steps of:

-   -   (i) protecting primary hydroxyl moiety of compound 2 with a         hydroxyl protecting agent to obtain hydroxyl protected compound         3;     -   (ii) converting the hydroxyl protected compound 3 into a borate         compound 4 using a borylation reagent via a transition metal         catalyzed cross-coupling reaction; and (iii) converting the         borate compound 4 into Tavaborole;     -   wherein the compounds 2, 3 and 4 and their substituents are as         described above for Scheme-1.

The borate compound 4 in step (iii) is converted into Tavaborole either by simultaneous deprotection of hydroxyl protecting group and boronate ester group using a hydroxyl deprotecting agent, or by sequential deprotection of hydroxyl protecting group and boronate ester group of compound 4 followed by cyclization in presence of an acid or a base.

The hydroxyl protecting agent comprises acyl protecting agent selected from the group consisting of acetyl chloride, acetic anhydride, and benzoyl chloride or Silyl protecting agents selected from the group consisting of trimethyl silyl chloride, triethyl silyl chloride, and tertiary butyl dimethyl silyl chloride (TBDMSCl).

The transition metal catalyzed cross-coupling reaction of step (ii) is carried out in presence of Pd catalyst selected from PdCl₂(dppf), [PdCl₂dppf]CH₂Cl₂, PdCl₂(PPh₃)₂, Pd(PPh₃)₄, Pd(OAc)₂, [Pd₂dba₃], [Pd(allyl)Cl]₂, Pd(acac)₂, PhPd(OAc)(PPh)₂ or any Palladium catalyst that generates in situ Pd(O) and combinations thereof or Ni catalyst selected from NiCl₂(dppp), NiCl₂(dppf), NiCl₂(dppe), NiCl₂.glyme, NiBr₂.glyme, NiCl₂(PPh₃)₂, NiCl₂(PCy₃)₂, Ni(PPh₃)₄, Ni(PPh₃)₄, Ni(COD)₂, NiCl₂, NiBr₂ or any Nickel catalyst that generates in situ Ni(O) and combinations thereof.

The borylation reagent is selected from the group consisting of Bis(neopentylglycolato)diboron, Bis(catecholato)diboron, Bis(hexyleneglycolato)diboron, Bis(pinacolato)diboron, Tetrahydroxydiboron, Pinacolborane, Methylpentanediolborane, Catecholborane, Neopentylglycoborane and Trialkyl borate.

The reaction of step (ii) is carried out in presence of a solvent selected from toluene, xylene, N,N-dimethylformamide, dimethylsufoxide, tetrahydrofuran and 1,4-dioxane or mixtures thereof.

The reaction of step (ii) is carried out in presence of a ligand selected from the group consisting of Xphos, MeO-CM-Phos, Sphos, DavePhos, RuPhos, tBu₃P-HBF₄, QPhos, JohnPhos, Me₄-tBu-XPhos, Ad₂PBu, BrettPhos, AmPhos, PPh₃ and tri(o-tolyl)phosphine.

The transition metal catalyst in step (ii) further comprises co-catalyst selected from the group consisting of Cu(OTf)₂, Cu(OAc)₂, CuCl, CuBr, CuI, ZnCl₂ and Ag₂CO₃.

The deprotection of hydroxyl protecting group is carried out by a hydroxyl deprotecting agent comprising an inorganic base selected from potassium hydroxide, lithium hydroxide and sodium hydroxide, metal carbonates selected from potassium carbonate, sodium carbonate and cesium carbonate, metal alkoxides selected from sodium methoxide and sodium ethoxide.

The deprotection of hydroxyl protecting group is carried out by a hydroxyl deprotecting agent comprising an acid deprotecting agent selected from HCl, H₂SO₄, HNO₃, AcOH, HCOOH, BF₃Et₂O, AlCl₃ and BBr₃.

The cyclization is carried out using an acid selected from the group consisting of HCl, H₂SO₄, HNO₃, AcOH, HCOOH, BF₃Et₂O, AlCl₃ and BBr₃.

In another embodiment, the invention provides a process for preparation of Tavaborole comprises the steps of:

-   -   (i) protecting primary hydroxyl moiety of a compound of 2a with         acetic anhydride in presence of DCM and pyridine to obtain         hydroxyl protected compound 3a;

-   -   (ii) converting the hydroxyl protected compound 3a obtained in         step (i) into a borate compound 4a using bis(pinacolato)diboron         via PdCl₂(dppf) catalyzed cross-coupling reaction in presence of         1,4-dioxane and potassium acetate (KOAc), wherein the reaction         is carried out at temperature between 70° C. to 120° C.;

-   -   (iii) simultaneous deprotection of hydroxyl protecting group and         boronate ester group of borate compound 4a obtained in step (ii)         by treatment with acid HCl or base NaOH to obtain Tavaborole.

In another embodiment, the invention provides a process for preparation of Tavaborole comprising following steps:

-   -   (i) protecting primary hydroxyl moiety of a compound 2b with         TBDMSCl in presence of dichloromethane (DCM) and imidazole to         obtain hydroxyl protected compound 3b;

-   -   (ii) converting the hydroxyl protected compound 3b obtained in         step (i) into a borate compound 4b using bis(pinacolato)diboron         via Pd(OAc)₂ catalyzed cross-coupling reaction in presence of         acetonitrile, Tri-O-tolylphosphine, Cs₂CO₃ and CuI;

-   -   (iii) treating compound 4b obtained in step (ii) with sodium         periodate (NaIO4) in presence of acetone, water and ammonium         acetate followed by further treatment with HCl to obtain         Tavaborole.

In another embodiment, the invention provides a process for preparation of 5-fluoro-1,3-dihydro-1-hydroxy-2,1-benzoxaborole (Tavaborole) comprising the steps of:

-   -   (i) converting a compound 2 into a borate ester compound 5 using         a borylation reagent via a transition metal catalyzed         cross-coupling reaction;     -   (ii) converting the borate ester compound of formula 5 obtained         in step (i) into Tavaborole;

The conversion of borate ester compound 5 into Tavaborole in step (ii) comprises the sub-steps of:

-   -   (i) oxidative cleaving of the borate ester compound of formula 5         into a compound of formula 6;     -   (ii) converting the compound of formula 6 into Tavaborole in         presence of an acid.

The above said compounds 2, 5 and 6 and their substituents are as described above for Scheme-2.

The transition metal catalyzed cross-coupling reaction of step (i) is carried out in presence of Pd catalyst selected from PdCl₂(dppf), [PdCl₂dppf]CH₂Cl₂, PdCl₂(PPh₃)₂, Pd(PPh₃)₄, Pd(OAc)₂, [Pd₂dba₃], [Pd(allyl)Cl]₂, Pd(acac)₂, PhPd(OAc)(PPh)₂ or any Palladium catalyst that generates in situ Pd(O) and combinations thereof; or Ni catalyst selected from NiCl₂(dppp), NiCl₂(dppf), NiCl₂(dppe), NiCl₂.glyme, NiBr₂.glyme, NiCl₂(PPh₃)₂, NiCl₂(PCy₃)₂, Ni(PPh₃)₄, Ni(PPh₃)₄, Ni(COD)₂, NiCl₂, NiBr₂ or any Nickel catalyst that generates in situ Ni(O) and combinations thereof.

The borylation reagent is selected from the group consisting of Bis(neopentylglycolato)diboron, Bis(catecholato)diboron, Bis(hexyleneglycolato)diboron, Bis(pinacolato)diboron, Tetrahydroxydiboron, Pinacolborane, Methylpentanediolborane, Catecholborane, Neopentylglycoborane and Trialkyl borate.

The reaction of step (i) is carried out in a solvent selected from MeOH, EtOH, IPA, toluene, xylene, N,N-dimethylformamide, dimethylsufoxide, tetrahydrofuran and 1,4-dioxane or mixtures thereof.

The reaction of step (i) is carried out in presence of a base comprising an inorganic base selected from the group consisting of KOPh, KOAc, NaOAc, NH₄OAc, Cu(OAc)₂, Cs₂CO₃, K₂CO₃, Na₂CO₃, Ag₂CO₃, K₃PO₄, NaOH, KOH, CsOH, KOMe, NaOMe, LiOtBu, NaOtBu and KOtBu or an organic base selected from the group consisting of pyridine, triethyl amine, leutidine, DABCO, DBU, 1,2,2,6,6-pentamethylpipiridine, 1,1,3,3-tetramethylguanidine, iPr₂NEt, NBu₃ and Cy₂NMe.

The reaction of step (i) is carried out in presence of a ligand selected from the group consisting of Xphos, MeO-CM-Phos, Sphos, DavePhos, RuPhos, tBu₃P-HBF₄, QPhos, JohnPhos, Me₄-tBu-XPhos, Ad₂PBu, BrettPhos, AmPhos, PPh₃ and tri(o-tolyl)phosphine.

The transition metal catalyst in step (i) further comprises co-catalyst selected from the group consisting of Cu(OTf)₂, Cu(OAc)₂, CuCl, CuBr, CuI, ZnCl₂ and Ag₂CO₃.

The oxidative cleaving in step (i) comprises treating the compound of formula 5 with NaIO4 or Pb(OAc)₄ in presence of a solvent selected from the group consisting of H₂O, tetrahydrofuran, 1,4-dioxane, methanol and ethanol or mixtures thereof.

The acid in step (ii) is selected from the group consisting of HCl, H₂SO₄, AcOH, HCOOH, BF₃Et₂O, AlCl₃ and BBr3.

The above said processes further comprises purification of the Tavaborole with solvents comprising alkanes consisting of Hexanes, Toluene and cyclohexane, alcohols consisting of Methanol, Ethanol and Isopropyl alcohol, water, Acetonitrile, Tetrahydrofuran, Acetone, Ethyl acetate, and Dichloromethane or mixtures thereof.

In one embodiment, the invention provides a process for purification of Tavaborole comprising the steps selected from:

-   -   (a) mixing Tavaborole in a solvent and treating the solution         with an anti solvent;     -   (b) mixing Tavaborole in a solvent, optionally heating the         solution to dissolve the compound and isolating the pure         compound by cooling the solution;     -   (c) treating Tavaborole with silica gel in a solvent followed by         recrystallization; and     -   (d) dissolving Tavaborole in aqueous alkali solution followed by         washing with Ethyl acetate, adjusting the pH of the aqueous         layer to acidic with HCl, and filtering the precipitated solid;     -   wherein the solvent in any of the above process is selected from         alkanes consisting of Hexanes, Toluene and cyclohexane, alcohols         consisting of Methanol, Ethanol and Isopropyl alcohol, water,         Acetonitrile, Tetrahydrofuran, Acetone, Ethyl acetate, and         Dichloromethane or mixtures thereof; and wherein the         anti-solvent is water.

In another embodiment, the invention provides process for purification of Tavaborole comprising the steps selected from:

-   -   (a) dissolving crude Tavaborole in Acetonitrile at 25-60° C. and         isolating by adding anti-solvent water to obtain the pure         product;     -   (b) dissolving crude Tavaborole in a mixture of IPA and toluene         at 40-80° C. and isolating by cooling the solution to obtain the         pure product;     -   (c) treating crude Tavaborole with silica gel in toluene and         recrystallizing in hexane to obtain the pure product; and     -   (d) dissolving crude Tavaborole in a mixture solution of Tolune         and acetonitrile at higher temperature, gradually cooling the         mass to precipitate the solid, filtering the obtained solid and         drying to obtain the pure product.

In another embodiment, the invention provides process for purification of Tavaborole to obtain purified Tavaborole with more than 99% purity comprising the steps of:

-   -   (a) dissolving crude Tavaborole in a solvent at 50-80° C.;     -   (b) cooling the solution to 0-5° C.; and     -   (c) filtering and drying to obtain purified Tavaborole with a         purity more than or equal to 99%;     -   wherein the solvent in step (a) is selected from a mixture of         IPA/ethyl acetate (3:4), a mixture of Toluene/IPA (5:1), a         mixture of Toluene/Acetonitrile (4:2), a mixture of         ethanol/water (1:5); and a mixture of Acetonitrile/water (1:10).     -   wherein the purified Tavaborole obtained by the process has more         than 99% purity and does not contain process impurities         represented as Impurity-A and Impurity-B.

In another aspect, the invention provides purified 5-fluoro-1,3-dihydro-1-hydroxy-2,1-benzoxaborole (Tavaborole 1) with greater than 99% purity and without the impurities represented as Impurity-A and Impurity-B.

In another aspect, the invention provides a process for preparation of crystalline forms of Tavaborole comprising the steps of:

-   -   (a) contacting Tavaborole with a solvent or mixture of solvents;     -   (b) heating the solvent or solvents mixture with Tavaborole to         reflux; and     -   (c) precipitating Tavaborole from solvent or solvents mixture by         using conventional techniques viz. slow or fast cooling,         addition of crystallization solvent, drying at atmospheric or         reduced pressure, so as to obtain the desired crystalline form         of Tavaborole;         -   wherein the solvent or solvents mixture can be selected from             water, methanol, Ethanol, Isopropyl alcohol, Methyl tertiary             butyl alcohol, Toluene, acetone Tetrahydrofuran, Ethyl             acetate, dichloromethane, acetonitrile, methyl tertiary             butyl ether or mixtures thereof; and         -   wherein the crystalline forms of Tavaborole produced by the             process is any of polymorphic forms Form-I, Form-II,             Form-III, Form-IV and Form-V, each polymorphic form is             characterized by XRPD peaks patterns as depicted in FIG. 1,             FIG. 3a and FIG. 3b , FIG. 4a and FIG. 4b , FIG. 5a and FIG.             5b , and FIG. 6a and FIG. 6b respectively.

The invention further provides:

-   -   Crystalline Form I of Tavaborole, characterized by X-ray powder         diffraction (XRPD) pattern comprising one or more of the         reflections at value 6.30, 12.66 and 25.5±0.2 degrees 2 theta         (2θ°) and which has X-ray powder diffraction pattern as shown in         FIG. 1; and further characterized by XRPD pattern comprising one         or more of the reflections at value: 5.70, 11.43, 14.25, 14.83,         16.33, 20.31, 26.23, 26.98, 28.84, 30.95, 32.06, 33.44, 36.96,         38.71, 41.40±0.2 degrees 2 theta (2θ°).     -   A process for preparing crystalline Form I of Tavaborole         comprising refluxing Tavaborole in a mixture of acetonitrile and         toluene and isolating Tavaborole by cooling to 0-5° C.     -   Crystalline Form II of Tavaborole characterized by X-ray powder         diffraction (XRPD) pattern comprising one or more of the         reflections at value 6.31, 14.35, 14.91, 16.44 and 27.08±0.2         degrees 2 theta (2θ°) and which has X-ray powder diffraction         pattern as shown in FIG. 3a and FIG. 3b ; and further         characterized by XRPD pattern comprising one or more of the         reflections at value: 12.68, 12.91, 20.41, 24.95, 25.51, 25.85,         26.33, 26.68, 27.08, 27.68, 28.93, 30.51, 31.74±0.2 degrees 2         theta (2θ°).     -   Crystalline Form III of Tavaborole characterized by X-ray powder         diffraction (XRPD) pattern comprising one or more of the         reflections at value 6.21, 12.55, 14.23, 14.82, 16.36, 25.37,         and 26.99±0.2 degrees 2 theta (2θ°) and which has X-ray powder         diffraction pattern as shown in FIG. 4a and FIG. 4b ; and         further characterized by XRPD pattern comprising one or more of         the reflections at value 5.56, 12.88, 20.27, 24.17, 24.83,         25.37, 26.23, 26.99, 28.81, 30.05, 30.33, 30.9, 31.9, 33.37,         36.88 and 38.56±0.2 degrees 2 theta (2θ°).     -   Crystalline Form IV of Tavaborole, characterized by X-ray powder         diffraction (XRPD) pattern comprising one or more of the         reflections at value: 6.35, 7.55, 8.25, 8.46, 14.2, 14.6, 14.9,         16.5, 25.46, 25.84, 26.38 and 26.63±0.2 theta degree (2θ°) and         which has X-ray powder diffraction pattern as shown in FIG. 5a         and FIG. 5b ; and further characterized by XRPD pattern         comprising one or more of the reflections at value 7.09, 10.23,         12.74, 13.17, 13.76, 15.27, 15.55, 17.07, 17.72, 18.15, 19.31,         19.90, 20.51, 21.88, 22.8, 23.08, 24.29, 24.92, 27.09, 27.67,         29.42, 30.19 and 31.66±0.2° 2θ.     -   Crystalline Form V of Tavaborole characterized by X-ray powder         diffraction (XRPD) pattern comprising one or more of the         reflections at 6.3, 7.48, 8.21, 8.37, 14.16, 14.35, 14.54,         14.88, 15.55, 16.41, 25.37, 25.78, 26.29, 26.5 and 27.03±0.2° 2θ         and which has X-ray powder diffraction pattern as shown in FIG.         6a and FIG. 6b ; and further characterized by XRPD pattern         comprising one or more of the reflections at value 7.01, 10.16,         10.68, 12.64, 13.14, 15.2, 16.07, 16.67, 17.05, 17.64, 18.08,         19.88, 20.43, 22.77, 24.3, 24.87, 26.29, 27.37, 28.90,         29.39±0.2° 2θ.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1: represents a characteristic X-Ray powder diffraction pattern of Tavaborole Form I

FIG. 2: represents a characteristic differential scanning calorimetry (DSC) of Tavaborole of Form I

FIG. 3a and FIG. 3b : represents a characteristic X-Ray powder diffraction pattern of Tavaborole Form II

FIG. 4a and FIG. 4b : represents a characteristic X-Ray powder diffraction pattern of Tavaborole Form III

FIG. 5a and FIG. 5b : represents a characteristic X-Ray powder diffraction pattern of Tavaborole Form IV

FIG. 6a and FIG. 6b : represents a characteristic X-Ray powder diffraction pattern of Tavaborole Form V

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to processes for the preparation of Tavaborole in significantly high yield and substantially pure form. The invention also comprises novel polymorphic forms of Tavaborole and processes for their preparation. The invention also provides processes for purification of Tavaborole to obtain substantially pure Tavaborole, comprising purity more than or equal to 99% and devoid of process related impurities represented as Impurity A and Impurity B.

The methods for the preparation of Tavaborole are schematically represented by Scheme-1 and Scheme-2.

One of the reported procedures of the invention (Scheme—1), comprises

-   -   i. Conversion of compound 2 into compound 3 using a suitable         hydroxyl protecting agent.     -   Non limiting examples of the suitable hydroxyl protecting         groups/agents are as defined under “definitions”. In one aspect         of the invention, the hydroxyl protecting group is the acyl         protecting agent preferably acetic anhydride and the reaction is         carried out in dichloromethane as solvent at room temperature in         the presence of catalytic amount of pyridine. In another aspect         of the invention, the hydroxyl protecting group is the silyl         protecting agent preferably TBDMSCl and the reaction is carried         out in THF solvent in the presence of imidazole base.     -   ii. Conversion of compound 3 into borate compound 4 via a         transition metal catalyzed cross-coupling reaction.     -   Non limiting examples of the borylation reagent, transition         metal catalyst, ligands and the solvent employed are as defined         hereunder. Compound 3 is treated with 1 to 2 equivalents of         borylation reagent in the presence of 1 to 4 mol % (0.01 to 0.04         equivalents with respect to compound 3, transition metal         catalyst, 1 to 5 equivalents of base and/or with the use of         appropriate ligand in a suitable solvent. Occasionally these         metal catalyst reactions may be conducted in the presence of         additive or co-catalyst. Reaction temperatures range from 70° C.         to 120° C., preferably 80° C. to 90° C. Reaction completion         times range from 10 minutes to 24 hrs, preferably 6-8 hrs.     -   Suitable solvent used for this reaction step is selected from         the group comprising toluene, xylene, N,N-dimethylformamide,         dimethylsufoxide, tetrahydrofuran, 1,4-dioxane or mixtures         thereof. In one aspect of the invention, the most preferred         solvent used for the reaction is 1,4-dioxane.     -   iii. Conversion of compound 4 into Tavaborole (1)         -   a) either by simultaneous deprotection of hydroxyl             protecting group and the borate group (ester or acid, when             R², R³=‘H’) of compound 4 using a suitable deprotecting             agent optionally in the presence of an acid or base; or         -   b) by sequential deprotection of hydroxyl protecting group             and boronate ester group under suitable conditions followed             by cyclization in the presence of an acid or a base, the             most preferred acid is HCl and the most preferred base is             NaOH.

Compound 4 can be converted to Tavaborole by an alternate route via the formation of 6A. The reaction stage includes oxidative cleavage of compound 4 by treating with 1 to 3 equivalents of NaIO₄ or Pb(OAc)₄ in a suitable solvent followed by dehydrative cyclization in the presence of an acid or a base. Suitable solvent is selected from the group comprising H₂O, tetrahydrofuran, 1,4-dioxane, methanol, ethanol or mixtures thereof.

Another route of synthesis of Tavaborole is as represented in Scheme—2 below:

wherein the substituents X, R², R³ are as defined above and the process comprises

-   -   i. Conversion of compound 2 into compound 5 via a Nickel         catalyzed cross-coupling reaction.

Compound 2 is treated with 1 to 2 equivalents of borylation reagent in the presence of 5 to 15 mol % (0.05 to 0.15 equivalents with respect to compound 2) of Nickel catalyst, 2 to 5 equivalents of base in a suitable solvent, optionally with the use of appropriate ligand.

Reaction temperatures may range from 25° C. to 95° C., preferably 35° C. to 40° C. Reaction completion times range from 1 hr to 24 hr, preferably 10 to 12 hr.

Suitable solvent is selected from the group comprising methanol, ethanol, IPA, N,N-dimethylformamide, dimethylsufoxide, tetrahydrofuran, 1,4-dioxane or mixtures thereof.

The non limiting examples of the Nickel catalyst, base and the ligand is as described hereunder.

-   -   ii. In step-2, compound 5 is converted into Tavaborole (1) under         acid or base mediated hydrolysis conditions.

The acid used for the hydrolysis is selected from the group comprising HCl, H₂SO₄, AcOH, HCOOH, BF₃Et₂O, AlCl₃, BBr₃ or the like and the base is selected from the group comprising NaOH, KOH, LiOH, Na₂CO₃, K₂CO₃, LiCO₃, CsCO₃, NaOMe, EtOMe.

-   -   iii. In step-3, alternatively the borate ester of compound 5 is         oxidatively cleaved by treating with 1 to 3 equivalents of NaIO4         or Pb(OAc)₄ in a suitable solvent. Suitable solvent is selected         from the group comprising H₂O, tetrahydrofuran, 1,4-dioxane,         methanol, ethanol or mixtures thereof.     -   iv. In step-4, compound 6 is converted into Tavaborole (1) under         acidic or basic conditions by dehydrative cyclization. Suitable         acid is selected from the group comprising HCl, H₂SO₄, AcOH,         HCOOH, BF₃Et₂O, AlCl₃, BBr3 or the like. Suitable base is         selected from the group comprising NaOH, KOH, LiOH, Na₂CO₃,         K₂CO₃, LiCO₃, CsCO₃, NaOMe, EtOMe.

Tavaborole obtained by the above two syntheses is 92-95% pure. When the reaction is performed as exemplified in examples 4 and 10 supra, Tavaborole obtained is substantially pure, with purity greater than or equal to 99% and hence doesn't require further purification. Tavaborole isolated from the instant processes or from any other process can be further purified from solvents selected from the group comprising alkanes like hexanes, toluene, cyclohexane; alcohols like methanol, ethanol, IPA; water; acetonitrile; tetrahydrofuran; acetone; ethyl acetate; dichloromethane or mixtures thereof.

Tavaborole can be purified by any of the following procedures:

-   -   i. Mixing it in any of the above said suitable solvents or         mixtures thereof and treating the thus obtained solution with an         anti-solvent, which is water in many of the cases;     -   ii. Mixing Tavaborole in any of the above said solvents or         mixtures, optionally heating the solution to dissolve the         compound and isolating the pure compound by cooling the         solution;     -   iii. Treating Tavaborole with silica gel in any of the above         suitable solvents followed by crystallization in any of the         solvents or mixtures thereof, and     -   iv. Dissolving Tavaborole in aqueous alkali solution followed by         washing with ethyl acetate, adjusting the pH of the aqueous         layer to acidic with HCl, and filtering the precipitated solid.

In one aspect crude Tavaborole is dissolved in acetonitrile at 25-60° C., preferably at 45-50° C., and isolation step is performed by adding the anti-solvent water.

In the second aspect, crude Tavaborole is dissolved in mixture of IPA and toluene at 40-80° C., preferably at 60-70° C., and the pure product is isolated by cooling the solution.

In another aspect crude Tavaborole is treated with silica gel in toluene and recrystallized in hexane.

In another aspect crude Tavaborole is recrystallized from a mixture of toluene and acetonitrile solvents, which comprises the steps of dissolving the crude Tavaborole in toluene—acetonitrile mixture while heating, followed by gradual cooling during which pure Tavaborole solid precipitates.

The major process impurities which are eliminated during the purification of Tavaborole according to the present invention are depicted below (A & B), thus obtaining Tavaborole in substantially pure form with a purity greater than 99%.

In one aspect of the invention, crystalline polymorphic forms I, II, III, IV and V of Tavaborole are reported whose X-ray diffraction pattern are represented by FIG. 1, FIG. 3a and FIG. 3b , FIG. 4a and FIG. 4b , FIG. 5a and FIG. 5b , and FIG. 6a and FIG. 6b respectively and whose 2-theta values are tabulated in Table—1, Table—2, Table—3, Table—4 and Table—5 respectively.

The polymorphic crystalline Form I of Tavaborole may produce an X-ray diffraction pattern comprising one or more of the following reflections: 6.30, 12.66 and 25.5, ±0.2° 2θ or that produces an X-ray powder diffraction pattern further comprising one or more of the following reflections: 5.70, 11.43, 14.25, 14.83, 16.33, 20.31, 26.23, 26.98, 28.84, 30.95, 32.06, 33.44, 36.96, 38.71, 41.40±0.2° 2θ or that produces an X-ray powder diffraction pattern comprising the 2 theta values tabulated in Table—1.

The polymorphic crystalline Form I of Tavaborole may also be identified by the DSC as depicted in FIG. 2. The X-ray diffraction pattern of Tavaborole polymorphic Form I is represented by FIG. 1.

The polymorphic crystalline Form II of Tavaborole may produce an X-ray diffraction pattern comprising one or more of the following reflections: 6.31, 14.35, 14.91, 16.44 and 27.08±0.2° 2θ or that produces an X-ray powder diffraction pattern further comprising one or more of the following reflections: 12.68, 12.91, 20.41, 24.95, 25.51, 25.85, 26.33, 26.68, 27.08, 27.68, 28.93, 30.51, 31.74±0.2° 2θ or that produces an X-ray powder diffraction pattern comprising the 2 theta values tabulated in Table—2. The X-ray diffraction pattern of Tavaborole polymorphic Form II is represented by FIG. 3a and FIG. 3 b.

The polymorphic crystalline Form III of Tavaborole may produce an X-ray diffraction pattern comprising one or more of the following reflections: 6.21, 12.55, 14.23, 14.82, 16.36, 25.37, and 26.99±0.2° 2θ or that produces an X-ray powder diffraction pattern further comprising one or more of the following reflections: 5.56, 12.88, 20.27, 24.17, 24.83, 25.37, 26.23, 26.99, 28.81, 30.05, 30.33, 30.9, 31.9, 33.37, 36.88 and 38.56±0.2° 2θ or that produces an X-ray powder diffraction pattern comprising the 2 theta values tabulated in Table—3. The X-ray diffraction pattern of Tavaborole polymorphic Form III is represented by FIG. 4a and FIG. 4 b.

The polymorphic crystalline Form IV of Tavaborole may produce an X-ray diffraction pattern comprising one or more of the following reflections: 6.35, 7.55, 8.25, 8.46, 14.2, 14.6, 14.9, 16.5, 25.46, 25.84, 26.38 and 26.63±0.2° 2θ or that produces an X-ray powder diffraction pattern further comprising one or more of the following reflections: 7.09, 10.23, 12.74, 13.17, 13.76, 15.27, 15.55, 17.07, 17.72, 18.15, 19.31, 19.90, 20.51, 21.88, 22.8, 23.08, 24.29, 24.92, 27.09, 27.67, 29.42, 30.19 and 31.66±0.2° 2θ or that produces an X-ray powder diffraction pattern comprising the 2 theta values tabulated in Table—4. The X-ray diffraction pattern of Tavaborole polymorphic Form IV is represented by FIG. 5a and FIG. 5 b.

The polymorphic crystalline Form V of Tavaborole may produce an X-ray diffraction pattern comprising one or more of the following reflections: 6.3, 7.48, 8.21, 8.37, 14.16, 14.35, 14.54, 14.88, 15.55, 16.41, 25.37, 25.78, 26.29, 26.5 and 27.03±0.2° 2θ or that produces an X-ray powder diffraction pattern further comprising one or more of the following reflections: 7.01, 10.16, 10.68, 12.64, 13.14, 15.2, 16.07, 16.67, 17.05, 17.64, 18.08, 19.88, 20.43, 22.77, 24.3, 24.87, 26.29, 27.37, 28.90, 29.39±0.2° 2θ or that produces an X-ray powder diffraction pattern comprising the 2 theta values tabulated in Table—5. The X-ray diffraction pattern of Tavaborole polymorphic Form V is represented by FIG. 6a and FIG. 6 b.

Generally, the crystalline forms of the present invention are prepared by contacting Tavaborole with a solvent, or mixture of solvents, one preferred method is refluxing Tavaborole in a mixture of solvents and then precipitating Tavaborole from the solvent using conventional techniques (e. g. slow or fast cooling, addition of crystallization solvent, drying at atmospheric or reduced pressure), so as to form the desired crystalline form Tavaborole.

The solvent components used in producing the crystalline forms of the present invention include methanol, ethanol, IPA, methyl tertiary butyl alcohol, toluene, acetone tetrahydrofuran, ethyl acetate, dichloromethane, acetonitrile or mixtures thereof.

According to the process of preparing crystalline Tavaborole Form I, the process comprising contacting Tavaborole with mixture of solvents; heating the solvents with Tavaborole to reflux; isolating Tavaborole by cooling the solvent to 0-5° C. and thereby obtaining the crystalline Tavaborole Form I. Suitable mixture of solvent for the preparation of crystalline tavaborole Form-I are selected from the group comprising ethyl acetate, toluene, acetonitrile, IPA.

In another embodiment,

-   -   the crystalline Form I of Tavaborole is obtained by refluxing         Tavaborole in a mixture of acetonitrile and toluene and         isolating Tavaborole by cooling to 0-5° C.;     -   the crystalline Form II of Tavaborole is obtained by the         crystallizing Tavaborole from toluene or mixture of toluene and         methanol;     -   the crystalline Form III of Tavaborole is obtained by the         crystallizing Tavaborole from water or mixture of water and         methanol;     -   the crystalline Form IV of Tavaborole is obtained by the         crystallizing Tavaborole from acetone or THF or melting of solid         Tavaborole Form I at 136° C. and cooling; and     -   the crystalline Form V of Tavaborole is obtained by         crystallizing Tavaborole from ethyl acetate, dichloromethane,         acetonitrile or methyl tertiary butyl ether solvents.

The X-Ray diffraction data of the Tavaborole crystalline forms obtained are tabulated below.

TABLE 1 X-ray diffraction data for Tavaborole Polymorphic Form I d value Peak no. Angle 2θ (°) Angstrom Intensity % 1. 5.70 15.46 4.7 2. 6.30 13.99 100 3. 11.43 7.73 1.7 4. 12.66 6.98 36 5. 14.25 6.20 13.5 6. 14.83 5.96 3.2 7. 16.33 5.42 4.8 8. 20.31 4.36 1.7 9. 25.52 3.48 26.5 10. 26.23 3.39 8.7 11. 26.98 3.30 12.1 12. 28.84 3.09 2 13. 30.95 2.88 3.3 14. 32.06 2.78 3.3 15. 33.44 2.67 2.3 16. 36.96 2.42 3.2 17. 38.71 2.32 6.5 18. 41.40 2.17 2.1

TABLE 2 X-ray diffraction data for Tavaborole Polymorphic Form II d value Peak no. Angle 2θ (°) Angstrom Intensity % 1. 6.31 13.97 30 2. 8.24 10.71 16.6 3. 8.43 10.47 7.1 4. 12.68 6.97 10.9 5. 12.91 6.85 5.6 6. 14.35 6.16 100 7. 14.91 5.93 27.5 8. 16.44 5.38 33 9. 20.41 4.34 5.6 10. 24.95 3.56 5.1 11. 25.51 3.48 10.4 12. 25.85 3.44 12.7 13. 26.33 3.38 10 14. 26.68 3.33 6.7 15. 27.08 3.28 14.7 16. 27.68 3.21 4.7 17. 28.93 3.08 9.5 18. 30.51 2.92 5.5 19. 31.74 2.81 5.1

TABLE 3 X-ray diffraction data for Tavaborole Polymorphic Form III d value Peak no. Angle 2θ (°) Angstrom Intensity % 1. 5.56 15.87 4.4 2. 6.21 14.21 68.9 3. 12.55 7.04 36.7 4. 12.88 6.86 5.1 5. 14.23 6.21 100 6. 14.82 5.97 38.6 7. 16.36 5.41 60.8 8. 20.27 4.37 10.9 9. 24.17 3.67 4.2 10. 24.83 3.58 6.5 11. 25.37 3.5 46.9 12. 26.23 3.39 14 13. 26.99 3.3 23.2 14. 28.81 3.09 12.3 15. 30.05 2.97 4.8 16. 30.33 2.94 9.2 17. 30.9 2.89 9.8 18. 31.9 2.8 5.2 19. 33.37 2.68 4.6 20. 36.88 2.434 7 21. 38.56 2.33 12

TABLE 4 X-ray diffraction data for Tavaborole Polymorphic Form IV Peak d value no. Angle 2θ (°) Angstrom Intensity % 1. 6.35 13.89 23.6 2. 7.09 12.45 9.6 3. 7.55 11.68 51.9 4. 8.25 10.7 95.7 5. 8.46 10.43 38.6 6. 10.23 8.63 9.3 7. 12.74 6.94 12.2 8. 13.17 6.71 6.1 9. 13.76 6.4 7.8 10. 14.2 6.2 100 11. 14.6 6.06 57.9 12. 14.91 5.9 29.7 13. 15.27 5.79 14.6 14. 15.55 5.69 10.1 15. 16.5 5.36 25.1 16. 17.07 5.18 12.4 17. 17.72 5 10.2 18. 18.15 4.88 9.3 19. 19.31 4.59 6 20. 19.9 4.45 19.3 21. 20.51 4.32 12.5 22. 21.88 4.05 4.2 23. 22.8 3.89 7.7 24. 23.08 3.84 8.7 25. 24.29 3.66 4.7 26. 24.92 3.56 9.8 27. 25.46 3.49 47 28. 25.84 3.44 59 29. 26.38 3.37 22 30. 26.63 3.34 33.1 31. 27.09 3.28 19.2 32. 27.67 3.22 18.4 33. 29.42 3.03 5.7 34. 30.19 2.95 5.1 35. 31.66 2.82 4.6

TABLE 5 X-ray diffraction data for Tavaborole Polymorphic Form V d value Peak no. Angle 2θ (°) Angstrom Intensity % 1. 6.3 14.01 29.7 2. 7.01 12.58 8.9 3. 7.48 11.8 56.2 4. 8.21 10.74 55.9 5. 8.37 10.55 22.2 6. 10.16 8.69 15.2 7. 10.68 8.27 4.9 8. 12.64 6.99 10.8 9. 13.14 6.72 7.9 10. 14.16 6.24 99 11. 14.35 6.16 100 12. 14.54 6.08 98.4 13. 14.88 5.94 34.8 14. 15.2 5.82 17.3 15. 15.55 5.69 24.6 16. 16.07 5.5 10.5 17. 16.41 5.39 42.2 18. 16.67 5.31 16.5 19. 17.05 5.19 8.2 20. 17.64 5.02 7.9 21. 18.08 4.9 14.2 22. 19.88 4.46 15.7 23. 20.43 4.34 11.4 24. 22.77 3.9 9.1 25. 24.3 3.65 7.4 26. 24.87 3.57 13.9 27. 25.37 3.5 52.4 28. 25.78 3.45 25.1 29. 26.29 3.38 26 30. 26.5 3.36 24.9 31. 27.03 3.29 32.3 32. 27.37 3.25 14.1 33. 28.9 3.08 9.5 34. 29.39 3.03 9

Definitions

The following terms shall have for the purpose of this application, including the claims appended hereto, the respective meanings set forth below:

“Hydroxyl or Hydroxy protecting groups”, the terms are used synonymously, means those groups that one skilled in the field would recognize as being suitable to protect the —OH substituent on an alkyl or ringed system as described herein and which may be removed under deprotection conditions known to those skilled in the field as set forth. Non-limiting examples of hydroxy protecting groups include ether protecting groups comprising benzyl ethers, silyl ethers, alkyl ethers including methyl ethers, ethyl ethers, propyl ethers, butyl ethers or the like; esters including benzoate, acetate or the like; acetals including MOP, BOM, THP or the like. Suitable acyl protecting agents used are selected from the group comprising of acetyl chloride, acetic anhydride, benzoyl chloride or the like; Suitable Silyl protecting agents used is selected from a group comprisisng of trimethyl silyl chloride, triethyl silyl chloride, and tertiary butyl dimethyl silyl chloride (TBDMSCl) or the like. In one aspect of the invention, the silyl protecting group employed is TBDMSCl.

Suitable Transition Metal catalysts that can be employed in the present invention are Pd catalyst or Ni catalysts. The Pd catalyst is selected from the group comprising of PdCl₂(dppf), [PdCl₂dppf].CH₂Cl₂, PdCl₂(PPh₃)₂, Pd(PPh₃)₄, Pd(OAc)₂, [Pd₂dba₃], [Pd(allyl)Cl]₂, Pd(acac)₂, PhPd(OAc)(PPh)₂ any Palladium catalyst that generates in situ Pd(0) and combinations thereof. In one aspect of the invention, Pd catalyst used is PdCl₂(dppf). Non limiting examples of Ni catalyst selected from group of NiCl₂(dppp), NiCl₂(dppf), NiCl₂(dppe), NiCl₂.glyme, NiBr₂.glyme, NiCl₂(PPh₃)₂, NiCl₂(PCy₃)₂, Ni(PPh₃)₄, Ni(COD)₂, NiCl₂, NiBr₂ or any Nickel catalyst that generates in situ Ni(0) and combinations thereof. In one aspect of the invention, Ni catalyst used is NiCl₂(dppp).

Appropriate ligand is selected from the group comprising of Xphos, MeO-CM-Phos, Sphos, DavePhos, RuPhos, tBu₃P-HBF₄, QPhos, JohnPhos, Me₄-tBu-XPhos, Ad₂PBu, BrettPhos, AmPhos, PPh₃, tri(o-tolyl) phosphine or the like. In one aspect of the invention, the ligand used is tri(o-tolyl)phosphine.

Suitable borylation reagent used is selected from the group comprising of boronic acids, Bis(neopentyl glycolato)diboron, Bis(catecholato)diboron, Bis(hexylene glycolato)diboron, Bis(pinacolato)diboron, Tetrahydroxydiboron, Pinacolborane, Methylpentanediolborane, Catecholborane, Neopentylglycoborane, Trialkyl borate. In one aspect of the invention, borylation reagent used is Bis(pinacolato)diboron.

Suitable base that can be employed in step 2 of Scheme—1 is an organic base or an inorganic base. Suitable inorganic base used is selected from the group comprising of KOPh, KOAc, NaOAc, NH₄OAc, Cu(OAc)₂, Cs₂CO₃, K₂CO₃, Na₂CO₃, Ag₂CO₃, K₃PO₄, NaOH, KOH, CsOH, KOMe, NaOMe, LiOtBu, NaOtBu, KOtBu or the like. In one aspect of the invention, the inorganic base used is KOAc. Suitable organic base used is selected from the group comprising of pyridine, triethyl amine, leutidine, DABCO, DBU, 1,2,2,6,6-pentamethylpipiridine, 1,1,3,3-tetramethylguanidine, iPr₂NEt, NBu₃, Cy₂NMe or the like. In one aspect of the invention, suitable organic base used is iPr₂Net.

“hydroxyl deprotecting agents or deprotecting agents” means those reagents that one skilled in the field would recognize as being suitable to remove the protecting groups of a hydroxyl moiety as described herein. Suitable hydroxyl deprotecting agents may be acid or base. Non-limiting examples being inorganic bases like potassium hydroxide, lithium hydroxide, sodium hydroxide, potassium carbonate, sodium carbonate or the like; metal alkoxides like sodium methoxide, sodium ethoxide; metal carbonates like sodium carbonate, potassium carbonate, cesium carbonate or the like; acid deprotecting agent selected from the group comprising HCl, H₂SO₄, HNO₃, AcOH, HCOOH, BF₃Et₂O, AlCl₃, BBr3 or the like.

These deprotecting agents are most suitable for simultaneously removing the boronate ester protecting groups.

Additive or co-catalyst that is employed is selected from the group comprising Cu(OTf)₂, Cu(OAc)₂, CuCl, CuBr, CuI, ZnCl₂, Ag₂CO₃ or the like.

The dehydrating agent used for the cyclization of Tavaborole is an organic or an inorganic acid selected from the group comprising HCl, H₂SO₄, HNO₃, AcOH, HCOOH, BF₃Et₂O, AlCl₃, BBr3 or the like. HCl being the most preferred acid. The base employed for cyclization of Tavaborole is selected from the group comprising NaOH, KOH, LiOH, Na₂CO₃, K₂CO₃, LiCO₃, CsCO₃, NaOMe, EtOMe.

Reagent Name Definition NBS N-Bromosuccinimide Pd Palladium MOP Methoxypropyl acetal BOP Benzyloxymethyl acetal TBDMSCl tert-Butyldimethylsilyl chloride n-BuLi n-Butyllithium BF₃Et₂O Boron trifluoride etherate NaIO₄ Sodium periodate Pb(OAc)₄ Lead tetraacetate [PdCl₂dppf]•CH₂Cl₂ [1,1′-Bis(diphenylphosphino)ferrocene]dichloro palladium(II), complex with dichloromethane PdCl₂(PPh₃)₂ Dichlorobis(triphenylphosphine)palladium(II) Pd(PPh₃)₄ Tetrakis(triphenylphosphine)palladium(0) Pd(OAc)₂ Palladium(II) acetate [Pd₂dba₃] Tris(dibenzylideneacetone)dipalladium [Pd(allyl)Cl]₂ Allylpalladium(II) chloride dimer Pd(acac)₂ Palladium(II) acetylacetonate PhPd(OAc)(PPh₃)₂ Bis(triphenylphosphine)phenyl palladium acetate PdCl₂(dppf) [1,1′-Bis(diphenylphosphino)ferrocene]dichloropalladium(II) NiCl₂(dppp) Dichloro[1,3-bis(diphenylphosphino)propane]nickelc NiCl₂(dppf) [1,1′-Bis(diphenylphosphino)ferrocene]dichloronickel(II) NiCl₂(dppe) 1,2-Bis(diphenylphosphino)ethane nickel(II) chloride NiCl₂•glyme dichloronickel; 1,2-dimethoxyethane NiBr₂•glyme Nickel(II) bromide 2-methoxyethyl ether complex NiCl₂(PPh₃)₂ Bis(triphenylphosphine)nickel(II) dichloride NiCl₂(PCy₃)₂ Bis(tricyclohexylphosphine)nickel(II) chloride Ni(PPh₃)₄ Tetrakis(triphenylphosphine)nickel(0) Ni(COD)₂ Bis(cyclooctadiene)nickel(0) NiCl₂ Nickel(II) chloride NiBr₂ Nickel(II) bromide Xphos 2-Dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl MeO-CM-Phos N-Methyl-2-(2′-dicylohexylphosphino-5′-methoxyphenyl)indole Sphos 2-Dicyclohexylphosphino-2′,6′-dimethoxybiphenyl DavePhos 2-Dicyclohexylphosphino-2′-(N,N-dimethylamino)biphenyl RuPhos 2-Dicyclohexylphosphino-2′,6′-diisopropoxybiphenyl tBu₃P-HBF₄ Tributylphosphonium tetrafluoroborate QPhos 1,2,3,4,5-Pentaphenyl-1′-(di-tert-butylphosphino)ferrocene JohnPhos (2-Biphenyl)di-tert-butylphosphine, (2-Biphenylyl)di-tert- butylphosphine Me₄-tBu-XPhos 2-Di-tert-butylphosphino-3,4,5,6-tetramethyl-2′,4′,6′-triisopropyl- 1,1′-biphenyl Ad₂PBu di(1-adamantyl)-n-butylphosphine BrettPhos 2-(Dicyclohexylphosphino)3,6-dimethoxy-2′,4′,6′-triisopropyl- 1,1′-biphenyl AmPhos Bis(di-tert-butyl(4-dimethylaminophenyl)phosphine) PPh₃ Triphenylphosphine KOPh Potassium phenoxide KOAc Potassium acetate NaOAc Sodium acetate NH₄OAc Ammonium acetate Cu(OAc)₂ Copper(II) acetate Cs₂CO₃ Caesium carbonate K₂CO₃ Potassium carbonate Na₂CO₃ Sodium carbonate Ag₂CO₃ Silver carbonate K₃PO₄ Tripotassium phosphate NaOH Sodium hydroxide KOH Potassium hydroxide CsOH Cesium hydroxide KOMe Potassium methoxide NaOMe Sodium methoxide LiOtBu Lithium tert-butoxide NaOtBu Sodium-t-butoxide KOtBu Potassium tert-butoxide DABCO 1,4-diazabicyclo[2.2.2]octane DBU 1,8-Diazabicyclo[5.4.0]undec-7-ene iPr₂NEt N,N-Diisopropylethylamine NBu₃ N-tributylamine Cy₂NMe N,N-Dicyclohexylmethylamine HNO₃ Nitric acid Cu(OTf)₂ Copper (II) triflate Cu(OAc)₂ Copper(II) acetate CuCl Copper(I) chloride CuBr Copper(I) bromide CuI Copper(I) iodide ZnCl₂ Zinc chloride AlCl₃ Aluminium chloride BBr₃ Boron tribromide Ag₂CO₃ Silver carbonate NaHCO₃ Sodium hydrogen carbonate LiCO₃ Lithium carbonate LiOH Lithium hydroxide Na₂SO₄ Sodium sulfate H₂O Water CH₂Cl₂ Dichloromethane MTBE Methyl tert-butyl ether IPA Isopropyl alcohol MeOH Methanol EtOH Ethanol EtOAc Ethyl acetate CCl₄ Carbon tetrachloride HCl Hydrochloric acid H₂SO₄ Sulfuric acid AcOH Acetic acid HCOOH Formic acid Ac Acetyl

The following examples further illustrate the present invention, but should not be construed in any way as to limit its scope.

Example-1 Preparation of 2-bromo-5-fluorobenzyl Acetate (3a)

Wherein in compound 2, X═Br and in compound 3, R¹=Ac

To a solution of (2-bromo-5-fluorophenyl)methanol (10.25 g, 0.05 mol) and acetic anhydride (7.1 mL; 0.075 mol) in dichloromethane (50 mL) two drops of pyridine is added and the mixture is stirred at room temperature until complete disappearance of starting material (reaction is monitored by TLC). After completion, the reaction mixture is quenched with water (100 ml), and the mixture is extracted by dichloromethane (120 mL). The organic layer is separated, and washed with saturated NaHCO₃ (100 mL), 0.1 N HCl (50 mL) and water (100 mL) and dried over Na₂SO₄. Evaporation of the solvent gave 2-bromo-5-fluorobenzyl acetate (Yield: 97%; Purity: 99.3%).

Example-2 Preparation of 5-fluoro-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl) Benzyl Acetate (4a)

Wherein in compound 4, R¹=Ac, R²& R³ together=borate ester

To a solution of 2-bromo-5-fluorobenzyl acetate (5 g, 0.02 mol) in 1,4-dioxane (60 mL), bis(pinacolato)diboron (5.6 g, 0.022 mol) and potassium acetate (5.6 g, 0.06 mol) are added. The resulting mixture is degassed using a stream of nitrogen and to this is added [1,1′-bis(diphenylphosphino) ferrocene]dichloropalladium(II) (250 mg, 0.34 mmol). The reaction mixture is then heated at 80° C. for 10 hrs. The mixture is cooled to room temperature, diluted with H₂O and extracted with EtOAc. The organic layer is dried over Na₂SO₄ and evaporated in vacuo to yield 5-fluoro-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl) benzyl acetate that is used in the next reaction step without further purification (Yield: 95%; Purity: 99.9%).

Example-3 Preparation of 5-fluoro-1,3-dihydro-1-hydroxy-2,1-benzoxaborole (Tavaborole 1)

A mixture of 5-fluoro-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl) benzyl acetate (3 g) and HCl (4N HCl, 30 mL) is stirred at 80° C. for 10 hrs. Water is added, and the mixture is extracted with ethyl acetate. The organic layer is washed with water (until water layer becomes neutral), dried over Na₂SO₄ and evaporated in vacuo to yield 5-fluoro-1,3-dihydro-1-hydroxy-2,1-benzoxaborole (Yield: 93%; Purity: 92%).

Example-4 Preparation of 5-fluoro-1,3-dihydro-1-hydroxy-2,1-benzoxaborole (Tavaborole 1)

To a solution of 5-fluoro-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzyl acetate (5 g) in Methanol (40 mL), NaOH (2 eq) is added and stirred for 4 hrs at RT. Solvent is removed at reduced pressure. The obtained crude is taken in mixture of THF (30 mL) and water (15 mL), and treated with concentrated HCl (7.5 ml). After completion of the reaction (usually completes in 12 hrs at RT), product is extracted with ethyl acetate and concentrated at reduced pressure to give the solid. The obtained compound is dissolved in aqueous NaOH solution (30 mL, 1 eq NaOH), washed with ethyl acetate, then acidified with aqueous HCl. The precipitated white solid is filtered, washed with water and dried (Yield: 80%; Purity: 99.8%).

Example-5 Preparation of 2-(acetoxymethyl)-4-fluorophenylboronic Acid (6A)

To a solution of 5-fluoro-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzyl acetate (5.13 g, 0.014 mol) in Acetone (50 mL), sodium periodate (8.9 g, 0.042 mol), ammonium acetate (2.4 g, 0.031 mol) in water is added. After stirring at room temperature for 10 hr, 2.5N HCl (40 mL) is added, and the mixture is stirred at 0-5° C. for 20 minutes. Water is added, and the mixture is extracted with ethyl acetate. The organic layer is washed with brine and dried on anhydrous sodium sulfate. The solvent is removed under reduced pressure, and the residue is treated with MTBE to afford 2-(acetoxymethyl)-4-fluorophenylboronic acid (Yield: 83%; Purity: 94%.

Example-6 Preparation of 5-fluoro-1,3-dihydro-1-hydroxy-2,1-benzoxaborole (Tavaborole 1)

A mixture of 2-(acetoxymethyl)-4-fluorophenylboronic acid (3 g) and HCl (4N HCl, 30 mL) is stirred at 80° C. for 10 hrs. Water is added, and the mixture is extracted with ethyl acetate. The organic layer is washed with water (until water layer becomes neutral), dried over Na₂SO₄ and evaporated in vacuo to yield 5-fluoro-1,3-dihydrobenzo[c][1,2]oxaborole (Yield: 93%; Purity: 92%).

Example-7 Preparation of (5-fluoro-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)methanol (5)

To a solution of (2-bromo-5-fluorophenyl) methanol (5 g, 0.02 mol) in 1,4-dioxane (60 mL), bis(pinacolato)diboron (5.6 g, 0.022 mol) and potassium acetate (5.6 g, 0.06 mol) were added. The resulting mixture was degassed using a stream of nitrogen and to this was added [1,1′-bis(diphenylphosphino) ferrocene]dichloropalladium(II) (250 mg, 0.34 mmol). The reaction mixture was then heated at 80° C. for 10 hrs. The mixture was cooled to room temperature, diluted with H₂O and extracted with EtOAc. The organic layer was dried over Na₂SO₄ and evaporated in vacuum to yield (5-fluoro-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)methanol that was used in the next reaction step without further purification. Yield-95%; Purity-99.9%.

Example-8 Preparation of (4-fluoro-2-(hydroxymethyl)phenyl)boronic Acid (6)

To a solution of (2-bromo-5-fluorophenyl) methanol (10.05 g, 0.049 mol) in Ethanol 100 mL), NiCl₂(dppp) (2.67 g, 0.0049 mol), (HO)₂B—B(OH)₂ (6.62 g, 0.074 mol), PPh₃ (0.01 mol), (DIPEA (25.7 mL, 0.148 mol) are added. The resulting mixture is degassed using a stream of nitrogen. The reaction mixture is stirred at reflux for 4 hrs, then cooled to room temperature, diluted with H₂O and extracted with EtOAc. The organic layer is dried over Na₂SO₄ and evaporated in vacuo to yield (4-fluoro-2-(hydroxymethyl)phenyl)boronic acid that is used in the next reaction step without further purification (Yield: 62%).

Example-9 Preparation of 4-fluoro-2-(hydroxymethyl)phenylboronic Acid (6)

Wherein compound-5, R²& R³=borate ester

To a solution of 5-fluoro-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)methanol (5.0 g, 0.02 mol) in Acetone (50 mL), sodium periodate (10.6 g, 0.05 mol), ammonium acetate (2.7 g, 0.035 mol) in water is added. After stirring at room temperature for 10 hr, the reaction mass is concentrated to give crude 4-fluoro-2-(hydroxymethyl)phenylboronic acid (Yield: 80%; Purity: 95%).

Example-10 Preparation of 5-fluoro-1,3-dihydro-1-hydroxy-2,1-benzoxaborole (Tavaborole 1)

A mixture of crude (4-fluoro-2-(hydroxymethyl) phenyl)boronic acid (3 g) and 10% H₂SO₄ solution (20 mL) is stirred at room temperature for 4 hrs. Water is added, and the mixture is extracted with ethyl acetate. The organic layer is washed with brine and dried on anhydrous sodium sulfate. The solvent is removed under reduced pressure, and the residue is treated with MTBE to afford benzo[c][1,2]oxaborole-1,5(3H)-diol (Yield: 81%; Purity: 95%).

Example-11 Preparation of 5-fluorobenzo[c][1,2]oxaborol-1(3H)-ol (Tavaborole 1)

To a solution of 5-fluoro-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl methanol (5 g) in 1,4-dioxane (30 mL) and water (15 mL), and treated with concentrated HCl (7.5 ml). After completion of the reaction (usually completes in 5-6 hrs at RT), product is extracted with ethyl acetate and concentrated at reduced pressure to give the solid. The obtained compound is dissolved in aqueous NaOH solution (30 mL, 1 eq NaOH), washed with ethyl acetate, then acidified with aqueous HCl. The precipitated white solid is filtered, washed with water and dried (Yield: 77%; Purity: 99.6%).

Example-12

Purification of Tavaborole (1)

a. Crude Tavaborole is treated with a mixture of IPA:ethyl acetate (3:4) at 60-65° C., then cooled to 0-5° C. sand maintained for 1 hr. Solid is filtered under vacuum, washed with 0.5 volumes of chilled 3:4 mixture of IPA and ethyl acetate solution. The resulting solid is dried under vacuum (Yield: 77%; Purity: 99.5%).

b. Crude Tavaborole is dissolved in mixture of toluene: IPA (5:1; 6 V) at 60° C., the obtained clear solution is slowly allowed to room temperature, then cooled to 0-5° C., filtered and dried (Yield 53%; Purity 99.2%).

c. Crude Tavaborole is dissolved in mixture of toluene: acetonitrile (4:2; 6 V) at 70-80° C., the obtained clear solution is slowly allowed to room temperature, then cooled to 0-5° C., filtered and dried (Yield 81%; Purity 99.9%).

d. Crude Tavaborole is treated with a mixture of ethanol: water (1:5) at 50-60° C., then cooled to 0-5° C. and maintained for 4 hrs. Solid is filtered, washed with 1 volume of chilled 1:5 mixture of IPA and water solution. The resulting solid is dried under vacuum (Yield: 78%; Purity: 99.7%).

e. Crude Tavaborole is dissolved in acetonitrile (2V) at 50° C. and water (20 V) is added drop wise. The solution is slowly allowed to reach room temperature, then cooled to 0-5° C., filtered, washed with a chilled mixture of acetonitrile: water; 1:10 (1V) and dried (Yield: 62%; Purity: 99.96%).

Example-13 Preparation of Tavaborole Polymorphic Forms

a. Preparation of Tavaborole Form I:

To 5.0 g of Tavaborole is added 13 ml of toluene and the mixture is heated at reflux conditions to attain clear solution. To this 1.0 ml of acetonitrile is added and cooled to 25-30° C. The solution is further cooled 0-5° C. and then stirred for 1.0 hr and the solid is filtered under vacuum, dried at 25-30° C. for overnight to get crystalline Tavaborole polymorphic Form I.

b. Preparation of Tavaborole Form II:

In 100 ml round bottom flask equipped with a reflux condenser, Tavaborole 2.0 g and toluene 5.0 ml are charged at 25-30° C., the solution is heated to 50-57° C. to get a clear solution and then cooled to 25-30° C. The solution is further cooled to 0-5° C. and the resulting solid (320 mg) is filtered, washed with chilled toluene and dried under vacuum to get crystalline Tavaborole Form II.

c. Preparation of Tavaborole Form III:

In 100 ml of round bottom flask equipped with a reflux condenser, Tavaborole 2.0 g and 10 ml of water are charged and heated to reflux at 97° C. The solution is cooled to 25-30° C. and further to 0-15° C. The precipitated solid is filtered (1.4 g) and washed with water, filtered and dried under vacuum to get crystalline Tavaborole Form III.

d. Preparation of Tavaborole Form IV:

In a 100 ml of round bottom flask equipped with a reflux condenser, Tavaborole (1.0 g) and 2.0 ml of acetone are charged and stirred to get clear solution. To this 18 ml of acetone is added and concentrated on rotatory evaporator. The solid that is scratched (0.62 g) from the wall of the round bottom flask after removal of entire solvent is crystalline Tavaborole Form IV.

The above Form IV can be obtained by melting polymorphic Form I of Tavaborole at 136° C. and cooling down to 25-30°.

e. Preparation of Tavaborole Form V:

In a 100 ml of round bottom flask, Tavaborole (1.0 g) and 3.0 ml of methyl tertiary butyl ether are charged and the mixture is stirred to get clear solution at 25-30° C. The solution is concentrated using rotatory evaporator to obtain Tavaborole as crystalline Form V.

Alternative Routes:

Example-14 Preparation of tert-butyl((5-fluoro-2-iodobenzyl) oxy) Dimethylsilane (3b)

Wherein in compound 2, X═I and in compound 3, R¹=TBDMS

To a solution of (5-fluoro-2-iodophenyl)methanol (3.78 g, 0.015 mol) in CH₂Cl₂ (40 mL), imidazole (3.0 g, 0.045 mol) and TBDMS chloride (2.5 g, 0.016 mol) are added successfully, the reaction mass is stirred at room temperature for 8 hrs. After completion, reaction mass is quenched with water, washed with brine solution, extracted with CH₂Cl₂, and the organic layer is dried over Na₂SO₄, and concentrated to give tert-butyl((5-fluoro-2-iodobenzyl)oxy)dimethylsilane (Yield-88%).

Example-15 Preparation of tert-butyl((5-fluoro-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzyl)oxy)dimethylsilane (4b)

Wherein in compound 4, R¹=TBDMS, R²& R³ together=borate ester

To a solution of tert-butyl((5-fluoro-2-iodobenzyl)oxy)dimethylsilane (4.76 g, 0.013 mol) in Acetonitrile (40 mL), Pd(OAc)₂ (292 mg, 0.0013 mol), Tri-O-tolylphosphine (395 mg, 0.0013 mol), Bis(pinacolato)diboron (4.98 g, 0.0196 mol), Cs₂CO₃ (6.39 g, 0.0196 mol), CuI (0.49 g, 0.0026 mol) are added. The resulting mixture is degassed using a stream of nitrogen. The reaction mixture is stirred at room temperature for 36 hrs. The mixture is cooled to room temperature, diluted with H₂O and extracted with EtOAc (120 mL). The organic layer is dried over Na₂SO₄ and evaporated in vacuo to yield tert-butyl((5-fluoro-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzyl)oxy)dimethylsilane and that is used in the next reaction step without further purification (Yield-87%).

Example-16 Preparation of 5-fluoro-1,3-dihydro-1-hydroxy-2,1-benzoxaborole (Tavaborole 1)

To a solution of tert-butyl((5-fluoro-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzyl)oxy)dimethylsilane (5.13 g, 0.014 mol) in Acetone (50 mL), sodium periodate (8.9 g, 0.042 mol), ammonium acetate (2.4 g, 0.031 mol) in water is added. After stirring at room temperature for 10 hr, 2.5N HCl (40 mL) is added, and the mixture is stirred at room temperature overnight. Water is added, and the mixture is extracted with ethyl acetate. The organic layer is washed with brine and dried on anhydrous sodium sulfate. The solvent is removed under reduced pressure, and the residue is treated with MTBE to afford 5-fluoro-1,3-dihydro-1-hydroxy-2,1-benzoxaborole (Yield: 83%; Purity: 94%). 

We claim:
 1. A process for preparation of a crystalline forms of Tavaborole comprising the steps of: (a) contacting Tavaborole with a solvent or mixture of solvents; (b) heating the solvent or solvents mixture with Tavaborole to reflux; and (c) precipitating Tavaborole from solvent or solvents mixture by using conventional techniques selected from slow or fast cooling, addition of crystallization solvent, drying at atmospheric or reduced pressure, so as to obtain the desired crystalline form of Tavaborole; wherein the solvent or solvents mixture can be selected from water, methanol, Ethanol, Isopropyl alcohol, Methyl tertiary butyl alcohol, Toluene, acetone Tetrahydrofuran, Ethyl acetate, dichloromethane, acetonitrile, methyl tertiary butyl ether or mixtures thereof; and wherein the obtained crystalline forms of Tavaborole are any of crystalline form Form-I, Form-II, Form-III, Form-IV and Form V.
 2. The crystalline Form I of Tavaborole as claimed in claim 1, characterized by X-ray powder diffraction (XRPD) pattern comprising one or more of the reflections at value 6.30, 12.66 and 25.5±0.2 degrees 2 theta (2θ°).
 3. The crystalline Form I of Tavaborole as claimed in claim 2, further characterized by XRPD pattern comprising one or more of the reflections at value: 5.70, 11.43, 14.25, 14.83, 16.33, 20.31, 26.23, 26.98, 28.84, 30.95, 32.06, 33.44, 36.96, 38.71, 41.40±0.2 degrees 2 theta (2θ°).
 4. A process for preparing crystalline Form I of Tavaborole as claimed in claim 2 comprising refluxing Tavaborole in a mixture of acetonitrile and toluene and isolating Tavaborole by cooling to 0-5° C.
 5. The crystalline Form II of Tavaborole as claimed in claim 1, characterized by X-ray powder diffraction (XRPD) pattern comprising one or more of the reflections at value 6.31, 14.35, 14.91, 16.44 and 27.08±0.2 degrees 2 theta (2θ°).
 6. The crystalline Form II of Tavaborole as claimed in claim 5 further characterized by XRPD pattern comprising one or more of the reflections at value: 12.68, 12.91, 20.41, 24.95, 25.51, 25.85, 26.33, 26.68, 27.08, 27.68, 28.93, 30.51, 31.74+0.2 degrees 2 theta (2θ°).
 7. The crystalline Form III of Tavaborole as claimed in claim 1, characterized by X-ray powder diffraction (XRPD) pattern comprising one or more of the reflections at value 6.21, 12.55, 14.23, 14.82, 16.36, 25.37, and 26.99±0.2 degrees 2 theta (2θ°).
 8. The crystalline Form III as claimed in claim 7 further characterized by XRPD pattern comprising one or more of the reflections at value 5.56, 12.88, 20.27, 24.17, 24.83, 25.37, 26.23, 26.99, 28.81, 30.05, 30.33, 30.9, 31.9, 33.37, 36.88 and 38.56±0.2 degrees 2 theta (2θ°).
 9. The crystalline Form IV of Tavaborole as claimed in claim 1, characterized by X-ray powder diffraction (XRPD) pattern comprising one or more of the reflections at value: 6.35, 7.55, 8.25, 8.46, 14.2, 14.6, 14.9, 16.5, 25.46, 25.84, 26.38 and 26.63±0.2 theta degree (2θ°).
 10. The crystalline Form IV of Tavaborole as claimed in claim 9 further characterized by XRPD pattern comprising one or more of the reflections at value 7.09, 10.23, 12.74, 13.17, 13.76, 15.27, 15.55, 17.07, 17.72, 18.15, 19.31, 19.90, 20.51, 21.88, 22.8, 23.08, 24.29, 24.92, 27.09, 27.67, 29.42, 30.19 and 31.66±0.2° 2θ.
 11. The crystalline Form V of Tavaborole as claimed in claim 1, characterized by X-ray powder diffraction (XRPD) pattern comprising one or more of the reflections at 6.3, 7.48, 8.21, 8.37, 14.16, 14.35, 14.54, 14.88, 15.55, 16.41, 25.37, 25.78, 26.29, 26.5 and 27.03±0.2° 2θ.
 12. The crystalline Form V of Tavaborole as claimed in claim 11 further characterized by XRPD pattern comprising one or more of the reflections at value 7.01, 10.16, 10.68, 12.64, 13.14, 15.2, 16.07, 16.67, 17.05, 17.64, 18.08, 19.88, 20.43, 22.77, 24.3, 24.87, 26.29, 27.37, 28.90, 29.39±0.2° 2θ. 